Q: Why is it not possible to have a CNAME and MX record for the same host name? For example, test.example.com cannot be both a CNAME and an MX record at the same time.
A: Unfortunately, what you’re running into is a limitation of the DNS specification. According to RFC 1034 “If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different.”
So, bottom line, this is not a limitation of our DNS platform, it is a limit of how DNS was written or designed many years ago. While we could adjust our code to allow it, it inevitably would confuse other DNS servers (resolvers) that are RFC compliant, thus we too must be entirely RFC compliant.
There is a potential workaround. You could use an ‘A’ record to point to an IP address instead of the CNAME. That works well with an MX record.